1. Field of the Invention
The present invention relates to a barium titanate semiconductive ceramic which has positive resistance-temperature characteristics and is suitable for applications such as controlling temperature and limiting current, or exothermic devices for constant temperature.
2. Background Art
A barium titanate semiconductive ceramic has positive resistance-temperature characteristics (PTC characteristics). It has low specific resistance at room temperature and its electric resistance suddenly rises when a particular temperature (the Curie temperature) is surpassed. Thus, the ceramic is widely used in a broad range of applications such as controlling temperature and limiting current, or in exothermic devices for constant temperature.
These applications require that titanate semiconductive ceramic have not only PTC characteristics, but also sufficiently withstand voltage to permit use under high voltage. Therefore, there is need for a semiconductive ceramic having PTC as well as high withstand voltage characteristics. Particularly, overcurrent-protection devices for circuits demand that higher withstand voltage be realized.
To this end, a variety of proposals have hitherto been made in order to enhance withstand voltage of barium titanate semiconductive ceramics. For example, Japanese Patent Application Laid-Open (kokai) No. 4-26101 discloses a ceramic having a ceramic grain size of 6-15 xcexcm, low specific resistance at room temperature and high withstand voltage obtained by adding Dy and Sb to a semiconductive ceramic comprising BaTiO3 and SrTiO3 incorporated with TiO2, SiO2, Al2O3, and MnO2. Japanese Patent Publication (kokoku) No. 60-25004 discloses that when a powder which is obtained by crushing, mixing and calcining a barium-titanium compounded oxalate and an Sb oxide (serving as a semiconducting agent) is fired at 1350xc2x0 C. while the calcining conditions and compacting pressure are controlled, there is obtained ceramic having a ceramic grain size of 1-5 xcexcm and high withstand voltage.
However, the semiconductive ceramic described in Japanese Patent Application Laid-Open (kokai) No. 4-26101 has a specific resistance of about 50 xcexa9cm and a withstand voltage of about 200 V/mm and such withstand voltage is still unsatisfactory. Also, the semiconductor material described in Japanese Patent Publication (kokoku) No. 60-25004 has a maximum withstand voltage of about 500 V/mm, which does not fully meet the current demand for enhancing withstand voltage.
Therefore, an object of the present invention is to provide barium titanate semiconductive ceramic having low specific resistance at room temperature and high withstand voltage, which fully meets the current demand for enhancing withstand voltage.
The present inventors have conducted earnest studies to attain the above object, and have found that a barium titanate semiconductive ceramic having an average ceramic grain size smaller than a certain value has low specific resistance at room temperature and excellent withstand voltage. The present invention was accomplished based on this finding.
Accordingly, the present invention provides a barium titanate semiconductive ceramic having an average ceramic grain size of about 0.9 xcexcm or less.
The barium titanate semiconductive ceramic of the present invention is preferably obtained from a barium titanate powder or a calcined barium titanate powder having strictly limited physical properties, i.e., a grain size of about 0.1 xcexcm or less, a cubic system crystal structure and a lattice constant of 4.02 angstroms or more, and containing a micro-quantity of a semiconducting agent in the form of solid solution.